In the manufacture of paper products, such as facial tissue, bath tissue, paper towels, dinner napkins and the like, a wide variety of product properties are imparted to the final product through the use of chemical additives. Examples of such chemical additives include softeners, debonders, wet strength agents, dry strength agents, sizing agents, opacifiers and the like. In many instances, more than one chemical additive is added to the product at some point in the manufacturing process. Unfortunately, there are instances where certain chemical additives may not be compatible with each other or may be detrimental to the efficiency of the papermaking process, such as can be the case with the effect of wet end chemical additives on the downstream efficiency of creping adhesives. Another limitation, which is associated with wet end chemical addition, is the need for the chemical additives to possess a charge, cationic, anionic or amphiphilic, but preferably cationic. The cationic charge is attracted to the anionic charge of the cellulose fibers allowing for the material to be retained on the cellulose fibers. Where anionic materials are used, a cationic promoter is required to retain the chemical on the fibers. Another limitation associated with wet end addition is the limited availability of adequate bonding sites on the cellulose papermaking fibers to which the chemical additives can attach themselves. Under such circumstances, more than one chemical functionality competes for the limited available bonding sites, oftentimes resulting in the insufficient retention of one or both chemical additives on the cellulose fibers.
A cellulose papermaking fiber primarily contains two types of functional groups, hydroxyl and carboxyl. At a typical papermaking pH of about 4 to about 9, a portion of the carboxyl groups are ionized causing the cellulose papermaking fibers to possess a net anionic charge. These anionic sites on the cellulose fibers serve as the source of attachment for wet end chemical additives. The amount of carboxyl groups on the cellulose fibers is limited and depends on the nature of the pulp. In general bleached kraft pulps contain about 2 to about 4 milli equivalents of carboxyl per 100 grams of pulp while mechanical pulps may contain upwards of about 30 to about 40 milli equivalents of carboxyl groups per 100 grams of pulp.
Most wet end chemical additives used in papermaking rely on ionic bonding for retention of the additive to the papermaking fibers. In general, the chemical additives will possess a positive charge somewhere on the molecule. The positive charge is attracted to the negative charge on the cellulose fibers and an ionic bond retains the chemical additives on the cellulose fibers. Where anionic chemical additives are used, a cationic promoter will be used to bridge the anionic chemical additive and the anionic sites on the cellulose fibers. The limited number of carboxyl groups on the cellulose fiber limits the amount of chemical additives that can be retained on the cellulose fibers. Also, where more than one chemical additive is used in the wet end, competition between the two chemical additives for the limited number of bonding sites on the cellulose fibers can result in inconsistent retention leading to variable product performance.
When added in the wet end, non-ionic chemical additives show poor retention to the cellulose papermaking fibers. An option to circumvent this issue is to covalently bond the molecule to the cellulose fibers in some way. A problem with covalent bonding to cellulose lies in the type of groups on the cellulose fibers that are available for reaction. The two chemically active groups on the cellulose fibers are hydroxyls and carboxyls. The carboxyl groups are generally too few in number and too low in reactivity to be useful. Also, any reaction at the carboxyl group will reduce the number of available ionic bonding sites on the cellulose fibers hence limiting the ability to retain any charged wet end chemical additives that may need to be used. The hydroxyl groups, while plentiful, are problematic in that anything that can react with a hydroxyl group can also react with water. In a typical papermaking process, on a molar basis, the amount of the hydroxyl groups on water available for reaction is magnitudes of order larger than the amount of the hydroxyl groups of the cellulose fibers available for reaction. Simple kinetics will therefore dictate a preference for reaction with water hydroxyl groups over the cellulose fiber hydroxyl groups. This problem can be overcome as exemplified with the sizing agents ASA (alkyl succinic anhydride) and AKD (alkyl ketene dimer). However, complicated and expensive emulsification must be performed in order to allow addition of these chemical additives to the wet-end of the process. The costs become prohibitively high for use in tissue. Additionally, such materials generally react with the hydroxyl groups of the cellulose fibers only after the forming process and removal of a majority of the water. Therefore the emulsions are cationic and the chemical additive is retained in the non-reacted state due to the attraction of the cationic emulsion for the anionic sites of the cellulose fibers. Hence, even in this case the amount of anionic sites on the cellulose fibers available for bonding with other charged wet end chemical additives is reduced.
Since softness and strength are both desirable traits in a tissue sheet, these traits are usually developed in combination within the tissue sheet by the addition of two separate chemical additives of the types described previously. A paper or tissue product normally contains, among other things, a mixture of hardwood and softwood cellulose fibers, as well as a chemical additive to increase strength and a chemical additive to increase softness. However, the way in which the softness and strength chemical additives bind to cellulose fibers is a problem. In order to be retained on the fibers, both softness and strength additives are usually cationic in nature, binding to the anionic sites in the cellulose fibers. Thus, the number of anionic sites on the cellulose fibers control the number of cationic molecules that can attach to the cellulose fibers. Most Kraft pulps typically contain only about 2 to about 3 milli-equivalents of anionic sites per 100 grams of cellulose fiber. However, the number of anionic sites actually on the surface of the fibers available for reacting with the chemical additives may be significantly lower.
Another problem with using positively charged paper modifying or debonder chemical additives is that these chemical additives must compete with cationic strength chemical additives for the limited anionic bonding sites on the cellulose fibers. This competition may result in unpredictable retention of the paper modifying and strength chemical additives, thereby providing a tissue product having varying softness and strength traits. In an alternative application, one of the chemical additives, typically the paper modifying chemical additive, can be incorporated to the tissue sheet after the sheet formation step by spraying or coating the chemical additive onto the tissue sheet. The chemical additive so applied then reacts with the anionic bonding sites on the surface of the cellulose fibers located on the surface of the tissue sheet. However, this approach can require the installation of the application equipment as well as engineering controls required to minimize airborne chemical exposure.
Another approach for treating paper or tissue products involves the covalent bonding of the chemical additives to the cellulose fibers. A problem with the covalent bonding of the chemical additives to the cellulose fibers resides in the type of groups available on the cellulose fibers that are available to react with the chemical additives. The two chemically active groups on the cellulose fibers are hydroxyl and carboxyl groups. The carboxyl groups are generally too few in number and too low in reactivity to be useful. The hydroxyl groups, while more plentiful, are problematic in that a chemical additive that is capable of reacting with a hydroxyl group is also capable of reacting with water. In a typical papermaking process, the amount of process water available for reaction is magnitudes of order larger than the amount of hydroxyl groups available for reaction on the cellulose fibers. As such, the majority of the known chemical additives will react with the process water rather than the hydroxyl groups available on the cellulose fibers. This problem can be reduced as demonstrated in the wet end application of sizing agents, such as ASA (alkyl succinic anhydride) and AKD (alkyl ketene dimer). However, complicated and expensive emulsification must be performed in order to allow addition of these sizing agents to the wet-end of the paper or tissue making process. The costs can become prohibitively high for use in paper and tissue products.
Because of the competition between the paper modifying chemical additives and the strength chemical additives for anionic bonding sites on the cellulose fiber, a better treatment result could be obtained if fewer chemical additives were used to complete the same tasks now performed by the addition of the paper modifying chemical additives and the strength chemical additives. The concurrent use of cationic strength chemical additives and cationic paper modifying chemical additives in the paper or tissue making process, as discussed above, does not give consistent ratios of the chemical additives in the finished paper or tissue products. In addition, these ratios may further depend on the pH, the temperature, and the order of addition of the chemical additives in the paper or tissue making process.
Therefore, there is a need for a means of retaining higher and more consistent levels of paper modifying chemical additives on the paper web via wet end addition. Furthermore, there is a need for retaining more than one chemical functionality to a paper web that mitigates the limitations created by the limited number of bonding sites.